Thermoplastic materials, such as hot melt thermoplastic adhesives, are conventionally stored and maintained in a solid state. When the material is to be used, it is melted and converted from a solid state to a molten state in a tank having heated walls. The melted material is maintained in the molten state within the tank and transmitted by a pump, as required, to one or more applicators or dispensers. If a particular application requires a substantial volume of hot melt thermoplastic material, a correspondingly large amount of material must be maintained in the molten or melted state within the tank and periodically replenished throughout a production run. Apparatus of this general type are disclosed, for example, in U.S. Pat. Nos. 3,964,645; 4,474,311; 4,667,850; 4,771,920; and, 4,821,922, all owned by the assignee of this invention.
Thermoplastic melting and dispensing apparatus of the type described above are effective in a number of applications, but can create environmental problems and degrade certain types of thermoplastic materials in some situations. A characteristic of most thermoplastic materials is that they oxidize, char or degrade when exposed to heat in the molten state over a prolonged length of time and/or when exposed to oxygen in the molten state. In relatively high throughput applications, and/or during a prolonged production run, the hoppers of apparatus of the type described above must be opened to allow additional solid thermoplastic material to be loaded therein as the quantity of material within the hopper becomes depleted. Removal of the top cover of the tank causes environmental problems since fumes or gaseous emissions are allowed to escape. In addition, the thermoplastic material within the tank is exposed to oxygen during the loading of additional material which can create charring or other degradation, particularly if a highly reactive thermoplastic material is employed.
One alternative to the tank or hopper-type melting and dispensing apparatus described above are bulk melter devices such as disclosed, for example, in U.S. Pat. Nos. 4,073,409; 4,240,567; and, 4,632,277, all owned by the assignee of this invention. Bulk melters of this type are designed for use with large, 55 gallon drums or barrels which contain thermoplastic material in the solid state. Such devices conventionally include a platen assembly comprising a follower, a heating platen located beneath the follower and a pump, all of which are insertable directly into the open top of the shipping drum. The heating platen is operable to melt the solid thermoplastic material in the region directly below the platen to convert that thermoplastic material into a liquid state for delivery to the pump. A seal is maintained between the follower of the platen assembly and the drum to prevent exposure of the thermoplastic material to atmosphere and to pressurize the contents of the drum. Because the contents of the shipping drum remain sealed until it is empty, escape of gaseous emissions and degradation caused by contact of the molten thermoplastic material with air are substantially eliminated.
One problem with bulk melters of the type described in the patents mentioned above is that they are not effective in applications where precise quantities of thermoplastic material must be supplied to applicators or dispensers. The pump associated with the platen assembly cannot be "dead-ended", i.e., the flow from the pump cannot be interrupted, which occurs, for example, when the thermoplastic dispensers or applicators are operated intermittently. As a result, bulk melters are typically used in combination with separate reservoirs or holding tanks wherein the molten thermoplastic material is transferred from the bulk melter to the holding tank, which, in turn, includes a pump to transmit the molten thermoplastic material to dispensers or applicators. While this arrangement reduces problems of degradation and charring of the thermoplastic material, the addition of a separate holding tank and pump for use with a bulk melter is relatively expensive and inefficient.